personally i didn t have the problem you listed before ( with listing the subcategories / for the saving new car attribute i haven t tested until now) : i installed the latest version of osclass 3.2.0 and after this the plugin car attribut using the market.
so i'm thinking perhaps your problem are coming from other plugins you have activated or from your theme or from precedent modification you ve made from the database .
i'm going to check if i can add new sub categorie for cars and i'll tell you if i get the same as you.
After checking : i can add new sub categories and new make without pb and osclass is saving them correctly.
so give us more information : theme used and plugins you activated
